Device for locating faults in cables



May 31, 1949. H. s. DIMMICK ETAL 4 DEVICE FOR LOCATING FAULTS IN CABLES5 Sheets-Sheet 1 Filed Dec. 22, 1943 May 31, 1949. H. s. DIMMICK ET AL2,471,417

DEVICE FOR LOCATING FAULTS IN CABLES Filed Dec. 22, 1943 5 Sheets-Sheet2 SOURCE Elma/"ma I Howard 5. Dimmick fis/fan 5T Jenkins WM WZ May 31,1949. H. .s. DIMMICK ETAL 2,471,417

DEVICE FOR LOGATING FAULTS IN CABLES Filed D60. 22, 1943 5 Sheets-Sheet3 0 Q I INVENTORS'.

HOWARD S. O/MMICK Fa -Ta s, TENN/N6 ATTORNEY May 31, 1949. H. s. DIMMICKEl'AL DEVICE FOR LOCATING FAULTS IN CABLES 5 Sheets-Sheet 4 Filed Dec.22, '1943 INVENTORS= l Ms M k 0 HF M 'M M ATTORNEY May 31, 1949. H, s,DIMMlcK ETAL 2,471,417

DEVICE FOR LOCATING FAULTS IN CABLES Filed Dec. 22, 1943 5 SheetsSheet 53 Wu cryiow Howard 5. D/rnm/clv Fe lion 5. Jnhzl 7Z6 device.

Patented May 31, 1949 DEVICE FOR LOCATING FAULTS 1N CABLES Howard S.Dimmick and Felton S. Jenkins, Philadelphia, Pa.

Application December 22, 1943, Serial No. 515,306

4 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) The invention described herein may be manufactured and usedby or for the Government for governmental purposes, without the paymentto us of any royalty thereon.

This invention relates to improvements in methods and devices forlocating faults in cables.

Heretofore, the location of a fault or failure in a cable depended uponthe use of either the Varley loop or the Wheatstone bridge method,

by means of which the approximate location of i the fault or failure isdetermined by calculation of the capacitance or resistance from someinitial point. These methods are objectionable in that they require toomuch time and mathematical calculations.

It is, therefore, an object of this invention to provide a method and adevice for rapidly locating faults in cables without requiringmathematical calculations.

A further object is the provision of an electronic device for detectingand locating faults in cables by producing a visual or audible signal.The exact location of the fault can be determined without requiring theremoval of plugs, connectors, insulation or shielding. Heretofore, therehas been no satisfactory method of determining the exact location of ashort in a cable without opening the insulation in numerous places.

A further object is the provision of a device for locating faults incables exactly and rapidly by visual means.

A further object is the provision of a device for locating faults incables, which device can be efficiently used by persons having noprevious electrical training.

These and other object are attained by the novel methods as well as theconstruction and arrangement hereinafter described and illustrated inthe accompanying drawings, in which:

Fig. 1 is a perspective view of a detecting unit used in the device.

Fig. 2 is a diagram showing the arrangement of units for locating shortcircuits.

Fig. 3 is a schematic wiring diagram of the detector unit.

Fig. 4 is a schematic diagram of a power supply unit.

Fig. 5 is a schematic diagram of an amplifying unit.

Fig. 6 is a diagram showing the magnetic fields and pick up coils for ashort circuit in unarmored cable.

Fig. 7 is a diagram showing the magnetic fields and pick up coils for ashort circuit in armored cable.

Referring to the drawings, in Fig. l is shown a detecting unit employedin the fault locating The detecting unit comprises a casing lDl, havinga cover I02 hinged thereto.

Posi- W tioned in the casing IN is a block I03 having a longitudinalgroove H14 through which the cable I l t passes. Hinged to block IE3 isa similar block 595 having a longitudinal groove N36 for the cable.Embedded in the blocks Hi3 and I and encircling the grooves IM and weare semi-circular non-magnetic metal bands ill! and H38 which have woundabout them respectively pick-up coils I29 and I 2 l, as shown in Fig. 4,forming sections of a toroid perpendicular to the axis of the grooves.These coils are relatively short and are placed on the supporting ringso that their magnetic axes are preferably displaced substantially Themagnetic pick-up in coils lZll and [2! depends on the dissymmetry of themagnetic field produced by currents in the cable conductors. It willtherefore be obvious that if the semi-circular pieces Mil and it? onwhich coils I20 and l2l are mounted are magnetic, the dissymmetry isdestroyed and no pick-up voltages will be obtained. This follows fromthe fact that in testing for short circuits the cable currents arealways equal and opposite and the resultant magnetomotive force actingaround a closed magnetic ring must be zero and hence no voltages will beinduced in coils wound on such a ring. However, if the pieces l0! and.I08 were short as compared with the circumference of the cable insteadof being semicircular, it would be possible to make these pieces ofmagnetic material.

The object of using two pick-up coils with axes substantially at rightangles is to avoid possible failure of the pick-up when only one coil isused. A single coil will failto pick up induced E. M. F. from currentswithin the cable when the plane of the conductors chance to benoninductively related to the pick-up coil. This right-anglerelationship of the two coils is merely the preferable one. If desired,the axes of the two coils may be at any other angle except to eachother. If they were placed in a 180 relationship, it would be possiblefor both coils to be non-inductive1y related to the current-carryingcable conductors.

Also positioned in the casing I lll is an electrode I09 not part of thepresent invention for detecting open circuits having a groove inalinement with the groove [04. The electrode N19 has hinged thereto agrooved cover IBSa. Supported by and projecting from the casing Hll' areguide troughs III which are arranged to be covered by similar guidetroughs H2 supported by the cover H12. Additional guide troughs H3 andcovers H4 are provided so that the cable llfl will be suitably supportedfor a considerable length.

Also mounted in the casing I BI is a change over switch element H5, anamplifier tube HE, and amplifier cable connectors I ll, H8, I IS, thepurpose of which will hereinafter be described.

In Fig. 3 is shown a schematic wiring diagram of the detector unit. Thecoils I20 and Ill are D sitioned at 90 to each, other and respectivelywound about the semi-circular non-magnetic metal bands I01 and I08. Theunit is connected to a dual channel amplifier unit I30, I3I shown inFig. 5 by the coaxial cable. connectors H0, H8. To balance the output ofthe coils I and RI, rheostats I22 and I23 are provided. Transformers I24, I25 are employed to match the input impedance of the dual channelamplifiers I30, ISTI shown in Fig. 2.

Short circuits are detected by means of the coils I20 and I2I, and opencircuits by means of the electrode I09. The output of the. transformersI24, I25 are fed through connectors H9, H8 to both of the dual channelamplifiers I30, I3I.

To operate the device it is necessary to provide a source of alternatingcurrent (see Fig. 4 of a controlled value preferably of about 5000-cycles. Any audio-frequency, however, may be used. The source I38 (seeFig. 4) includes a power supply unit I39, an oscillator I40, a variableamplifier MI, and a power amplifier stage I42. Means I43 are provided tomaintain the. current output relatively constant while locating shortcircuits.

The rectified current from the power supply unit I39 is controlled bythe voltage regulator tubes I44, one portion going to the oscillator I40to produce the desired 5000 cycle frequency. Another portion providesplate excitation for amplifiers MI and I42. which amplify the oscillatoroutput, which then passes through constant current regulator I43 toleads I41 and I48.

While an alternating current source is preferably used for supplyingcurrent to the cable, as is well known in the art it is necessary onlythat this source be one of variable current. For example, interrupteddirect current could be used.

In the dual channel amplifier unit (see Fig. 5), each channel has threeamplifier stages and a rectifier unit I32. A power'supply unit I31 and arelay I are common to both channels.

In operation, to locate a short circuit, an alternating current of anyaudio-frequency, such as 5000 cycles, is fed through the defectiveconductor III], by means of rotating slip rings and brush holder 5 (seeFig. 2). The current upon reaching the point of short returns throughthe conductor or shield by which the defective conductor is shortcircuited. The cable is moved from left to right through the axis of thetoroid outlined by the coils I20 and IE1. While the defeet is at theleft of the coils, an alternating current is induced in the coils and isamplified by the amplifiers I30, I3I'. The outputs of the amplifiers arerectified by rectifiers I32 and passed through an indicating instrumentI34 and a relay I35, which causes a green light I36 to be lighted (seeFig. 2). cable has moved to the right of coils I20, IZI, there is nolonger an alternating current induced in the coils. Consequently. theoutput of the amplifiers falls to zero, thereby releasing the armatureof relay I35. green light I36 and closes a circuit through red light I31. Power for these units is supplied by a power unit I39.

As stated above, for the short circuit test, the output of the coils ispassed through the two amplifiers I30, I 3|, each of which includesamplifier tubes I'5I, I32, I52. From tube I52 the amplifier current isreturned to tube I32 which then acts as a rectifier. The rectifiedcurrent then passes through an indicating meter I34 and When the deflectin the.

This opens the circuit to relay I35 to control the lights as abovedescribed.

The voltages induced in coils I 20, I 2| which are positioned at 90 toeach other, are amplified by two separate amplifiers I30 and I3I. Theexisting output of the amplifiers indicates that current is flowing inthe part of the cable that is in the center of the toroid formed by thecoils I20, Hi. When the cable is moved to such a position that thisoutput decreases or ceases, there is an indication that the point of thecable which is shorted has passed the plane of the toroid. If thecurrent decreases and does not cease entirely, this would indicate thatthere are more than one short circuit in the cable. Entire cessation ofthe current indicates that there is only one short. The cables aregenerally formed by two twisted wires. At certain axial angles to thecables the magnetic flux may be cancelled, and for this reason the twocoils are positioned at to each other. Two separate amplifiers are usedso that the sum of the voltages can beregistered regardless of the axialposition of the defective section of the conductor.

When there is more than one short circuit or ground in a conductor, adecided decrease in a1- ternating current induction pickup is noted whenthe first ground or short reaches the coil. Sufficient, however, ispresent in the cable to supply current to the next short circuit orground, Induction pickup by the coil will occur until the last shortcircuit or ground is reached. The outpu of h amplifiers en rgiz s arelay, whi h in turn lights a green lamp.

The action of the magnetic pick-up coils for locating short circuits wilno be d s ed in greater detail.

Ref r ng t F 6, two representa ve. c ductors of an unarmoredmulti-conductor cable are indicated in cross section at 200 and ZQI, theouter limits of the body insulation being indicated at 202. If theconductors are short circuited by a fault and energized with alternatingcurrent, magnetic fields are produced as inclicated by the lines offorce 203 for the instant the current directions are as shown.

If one of the pick-up coils, say I20, happens to lie as shown in thefigure, t e magnetic flux 23 threads through the turns of the coil and avoltage is induced which when amplified and rectified gives anindication on meter I34, Fig. 2 a F 5. If ne y chanc t e p ck-up coil ison the magnetic axis as indicated at I2I, it is evident that no fluxcuts through the coil in a manner to generate an induced voltage. Ifonly one coil is used, there are numerous places along the cable, due tothe twisting of the strands, where no pickup is obtained, any one ofwhich might be interpreted as the location of the short.

To obviate this difficulty, it has been found satisfactory to providetwo coils located at right angles as shown. Thus when coil I'ZI fails topick up an induced voltage, coil I20 is in its most sitestive positionand vice versa. If coils I20 and I2I are joined in series and a singleamplifier used, a position of the cable may be found where the resultantinduced voltage is again zero and a false indication obtained. It isnecessary therefore to provide separate amplifiers and rectifiers foreach pick-up coil to avoid risk of false dead spots.

In the case of an armored cable the same lgind of action takes place butthe explanation is not so evident. If the sheath were perfectlyconductins, according t acc pt d e ectrical theor n fields of any kind,electric or magnetic, could get out of the cable. Experiment, however,shows that fields, both electric and magnetic, do get out through thesheath. The explanation of this fact lies in the lack of perfectconduction in the sheath.

Referring now to Fig. 7, the cable is inclosed in a conducting sheath205, the other designations being the same as in Fig. 6. The lines ofmagnetic force would, if they could, follow the same paths as indicatedin Fig. 6. These fluxes Would have to pass through the conductingsheath, and in so doing, generate voltages in the sheath. These inducedvoltages produce currents in the sheath which, if perfectly conducting,would exactly neutralize all magnetomotive force outside the sheath. Inother words, the sheath currents neutralize all magnetic fluxes outsidethe sheath and confine it within the sheath. These sheath currents arein phase op- I therefore do not quite neutralize the exciting currentsso that some magnetic flux does get through and although greatly reducedmay be picked up by the same pick-up coils as those described. Forexample feeble magnetic flux 208 threads the turns of the pick-up coilI20, Fig. 7, similar to flux 203 of Fig. 6. A neutral position I2 I,Fig. 7, may also be found the same as in Fig. 6. These feeble pick-upvoltages require considerable amplification which is provided for by thethreestage amplifier shown in Fig. 5.

Having described our invention, what we claim 1. A system for locatingfaults in a cable comprising, a source of potential varying with time,means for applying said potential between the separate conductors of thecable, fault locating means including two pick-up coils assembled on anon-magnetic ring-shaped core surrounding the cable and spaced along theperiphery of the ring so that the angle between the magnetic axes issubstantially a right angle, means for moving the cable relative to andin inductive relation to the pick-up coils, separate amplifying meansconnected to the output of each coil, rectifying means connected to theoutput of each amplifying means and indicating means responsive to thesum of the outputs of the rectifying means.

2. A system for locating faults in a cable comprising, a source ofvarying potential, means for applying said potential between theseparate conductors of the cable, fault locating means having a cablerecess to permit axial motion of the cable therethrough, saidfault-locating means including two pick-up coils having magnetic axes ina plane substantially at right angles to the cable recess and to eachother so that at all times when a current carrying portion of the cableis adjacent to the coils a potential will be electromagnetically inducedin at least one of the coils, means for moving the cable relative to thefault-locating means, separate amplifying means connected to the outputof each coil, rectifying means connected to the output of eachamplifying means, and indicating means responsive to the sum of theoutputs of the rectifying means.

3. A system for locating faults in a cable comprising, a source ofvarying potential means for applying said potential between the separateconductors of the cable, fault locating means having a cable recessthrough which the cable may be moved, said fault-locating meansincluding two pick-up coils which are relatively short as compared withthe circumference of the cable, the magnetic axes of the coils lying ina plane substantially at right angles to the cable recess and being atan angle other than 180 or exact coincidence to each other, means formoving the cable relative to the fault-locating means, separateamplifying means connected to the output of each coil, rectifying meansconnected to the output of each amplifying means, and indicating meansresponsive to the sum of the outputs of the rectifying means.

4. A system for locating faults in a cable comprising, a source ofvarying potential, means for applying said potential between theseparate conductors of the cable, fault-locating means having a cablerecess through which the cable may be moved axially, said fault-locatingmeans including two pick-up coils which are relatively short as comparedwith the circumference of the cable, the magnetic axes of the coilslying in a plane substantially at right angles to the cable recess andbeing at right angles to each other, means for moving the cable relativeto the faultlocating means, separate amplifying means connected to theoutput of each coil, rectifying means connected to the output of eachamplifying means, and indicating means responsive to the sum of theoutputs of the rectifying means.

HOWARD S. DIMMICK. FELTON S JENKINS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

